ALTER TABLE ReturnProduct DROP rfk_ReturnProduct_Bill_FKID

ALTER TABLE ReturnProduct DROP COLUMN BillId

ALTER TABLE ReturnProductDetail DROP rfk_BReturnProductDetail_BillDetail_FKID
ALTER TABLE ReturnProductDetail DROP COLUMN BillDetailId

ALTER TABLE ReturnProductDetail ADD ProductId INT NULL

ALTER TABLE dbo.ReturnProductDetail
ADD CONSTRAINT rfk_ReturnProductDetail_Product_FKID FOREIGN KEY
    ( ProductId ) 
REFERENCES dbo.Product
    ( Id )
ON DELETE CASCADE